1. Field of the Invention
This invention relates to detection of failure conditions in high power electrical switching devices, particularly to the detection of high pressure conditions in a vacuum interrupter through the use of sonic transducers.
2. Description of the Related Art
The reliability of the North American power grid has come under critical scrutiny in the past few years, particularly as demand for electrical power by consumers and industry has increased. Failure of a single component in the grid can cause catastrophic power outages that cascade throughout the system. One of the essential components utilized in the power grid are the mechanical switches used to turn on and off the flow of high current, high voltage AC power. Although semiconductor devices are making some progress in this application, the combination of very high voltages and currents still make the mechanical switch the preferred device for this application.
There are basically two configurations for these high power mechanical switches; oil filled and vacuum. The oil filled switch utilizes contacts immersed in a hydrocarbon based fluid having a high dielectric strength. This high dielectric strength is required to withstand the arcing potential at the switching contacts as they open to interrupt the circuit. Due to the high voltage service conditions, periodic replacement of the oil is required to avoid explosive gas formation that occurs during breakdown of the oil. The periodic service requires that the circuits be shut down, which can be inconvenient and expensive. The hydrocarbon oils can be toxic and can create serious environmental hazards if they are spilled into the environment. The other configuration utilizes a vacuum environment around the switching contacts. Arcing and damage to the switching contacts can be avoided if the pressure surrounding the switching contacts is low enough. Loss of vacuum in this type of interrupter will create serious arcing between the contacts as they switch the load, destroying the switch. In some applications, the vacuum interrupters are stationed on standby for long periods of time. A loss of vacuum may not be detected until they are placed into service, which results in immediate failure of the switch at a time when its most needed. It therefore would be of interest to know in advance if the vacuum within the interrupter is degrading, before a switch failure due to contact arcing occurs. Currently, these devices are packaged in a manner that makes inspection difficult and expensive. Inspection may require that power be removed from the circuit connected to the device, which may not be possible. It would be desirable to remotely measure the status of the pressure within the switch, so that no direct inspection is required. It would also be desirable to periodically monitor the pressure within the switch while the switch is in service and at operating potential.
It might seem that the simple measurement of pressure within the vacuum envelope of these interrupter devices would be adequately covered by devices of the prior art, but in reality, this is not the case. A main factor is that the switch is used for switching high AC voltages, with potentials between 7 and 100 kilovolts above ground. This makes application of prior art pressure measuring devices very difficult and expensive. Due to cost and safety constraints, complex high voltage isolation techniques of the prior art are not suitable. What is needed is a method and apparatus to safely and inexpensively measure a high pressure condition in a high voltage interrupter, preferably remote from the switch, and preferably while the switch is at operating potential. Additionally, it is desirable to have a method and apparatus that can be retrofitted to existing switching devices without extensive re-work or de-commissioning, and which does not require the vacuum interrupter module to be removed from the insulation and packaging of the switch housing.
FIG. 1 (Prior Art) is a cross sectional view 100 of a first example of a vacuum interrupter of the prior art. This particular unit is manufactured by Jennings Technology of San Jose, Calif. Contacts 102 and 104 are responsible for the switching function. A vacuum, usually below 10−4 torr, is present near the contacts in region 114 and within the envelope enclosed by cap 108, cap 110, bellows 112, and insulator sleeve 106. Bellows 112 allows movement of contact 104 relative to stationary contact 102, to make or break the electrical connection.
FIG. 2 (Prior Art) is a cross sectional view 200 of a second example of a vacuum interrupter of the prior art. This unit is also manufactured by Jennings Technology of San Jose, Calif. In this embodiment of the prior art, contacts 202 and 204 perform the switching function. A vacuum, usually below 10−4 torr, is present near the contacts in region 214 and within the envelope enclosed by cap 208, cap 210, bellows 212, and insulator sleeve 206. Bellows 212 allows movement of contact 202 relative to stationary contact 204, to make or break the electrical connection.
FIG. 3 (Prior Art) is a partial cutaway view of an example VBM (vacuum breaker module) 300 containing a vacuum interrupter 302. The VBM module 300 has an outer insulation covering 304 and an inner insulation layer 306 to isolate the high voltage being switched by the vacuum interrupter 302. Such modules are commonly used throughout the power generation and distribution system for switching purposes, and are manufactured by, for example, Joslyn High Voltage Company of Cleveland, Ohio.
FIG. 4 (Prior Art) is a cross sectional view of an example VSV (Versa-Vac) capacitor switching module 400 containing a vacuum interrupter 402. The VSV module 400 has an outer insulation covering 406 and an inner insulation layer 404 to isolate the high voltage being switched by the vacuum interrupter 402. Such modules are also commonly used throughout the power generation and distribution system for switching purposes, and are manufactured by, for example, Joslyn High Voltage Company of Cleveland, Ohio.
As can be seen from the configurations of modules 300 and 400, accommodating a modified interrupter 302 or 402 may require extensive design changes to the insulation layers and packaging. It would be desirable to have a pressure detection means that is able to determine the pressure inside interrupters 302 or 402 without extensive modification of the outer insulation and packaging, which would enable retrofit of the large number of switches currently operating in the field. This would improve the reliability of the power generation and distribution systems without the costly replacement of currently installed vacuum interrupters.
U.S. Pat. No. 3,983,345 discloses a method of detecting a leak in any one of the vacuum-type circuit interrupters of a high voltage vacuum circuit breaker comprising a plurality of normally series-connected interrupters located within a tank of the circuit breaker containing pressurized gas. Through small openings in the wall of the tank, a first set of conductive rods are inserted to make electrical connection with predetermined terminals of the interrupters. Through other small openings in the tank wall, a second set of conductive rods, insulated from the tank wall, are inserted to make electrical connection with predetermined other terminals of the interrupters. These predetermined terminals are such that the interrupters are connected electrically in parallel between the first and second sets of rods. Between said first and second sets of rods a test voltage is applied to the interrupters in parallel that is of sufficient value to produce a high probability of dielectric breakdown within any interrupter stressed by said voltage that has lost its vacuum, thus providing an indication of such a loss of vacuum.
U.S. Pat. No. 4,103,291 discloses a leak sensor powered directly by the circuit voltage being controlled by the vacuum circuit interrupter and continuously operating while the interrupter is in service. An indicating system is connected to the leak sensor, or sensors, and provides an indication of failure and corrective action to be taken in single phase or multi-phase circuits.
U.S. Pat. No. 4,163,130 discloses a vacuum interrupter with pressure monitoring means wherein a pair of separable electrodes are arranged within a highly evacuated envelope and are connected to a high voltage circuit provided with a vacuum pressure detector element which has a pair of detector electrodes insulated from each other and serving to detect the pressure of the vacuum within the evacuated envelope. The vacuum pressure detector element has a voltage applied thereto in such a manner that one of the detector electrodes is conductively connected to the one end of the evacuated envelope to which the high voltage circuit is connected and the other detector electrode is connected to ground potential through a series connection member consisting of different sorts of voltage allotment elements which are selected from a resistance, an inductance, and a capacitor and whose voltage allotment ratio varies in dependence on frequency. A vacuum pressure detector means detects the operation of the vacuum pressure detector element.
U.S. Pat. No. 4,270,091 discloses a partial pressure gauge utilizes an efficient electron collision excitation source yielding de-excitation radiation characteristic of residual gases. The intensity of a given spectral line is proportional to the partial pressure of the gas having such spectral line, and the current drawn from the excitation source provides a measure of the total pressure. A calibration technique based upon comparing the emitted light intensity with the ion currents associated with the excitation process yields an accurate measure of the relative partial pressure. Use of a filter to selectively pass radiation from a known constituent in known proportion in ambient gas provides an indication of the presence of a leak without the need for probing with a test gas. Provision for passing an evaporant stream through the excitation region permits accurate monitoring of the evaporant flux from which deposition rate is determined. In combination with techniques for achieving high differential sensitivity to fluctuations in light output from a selected spectral line, a novel leak detector is achieved. In combination with an optically dispersive element a residual gas analyzer is obtained.
U.S. Pat. No. 4,402,224 discloses a monitoring device for monitoring vacuum pressure of an electrical device employing an evacuated envelope. The patent discloses, particularly, a pressure responsive monitoring device which comprises an electric field generating device of vacuum type, an electric field detector means including a light source for generating light, an electric field detector detecting change of the electric field of the electric field generator due to the change of vacuum pressure inside the envelope and controlling the light depending upon the change of the electric field, and photoelectric converter for converting the light controlled by the electric field detector to an electric signal which is employed to monitor the vacuum pressure of the envelope.
U.S. Pat. No. 4,403,124 discloses a vacuum circuit interrupter which utilizes the vapor deposition shields thereof in the existing high voltage source or network which is controlled by the circuit interrupter to produce a cold cathode ion detector for determining the quality or amount of vacuum within the vacuum circuit interrupter. The central shield support ring which protrudes through the insulating casing of the circuit interrupter is used to supply ion current to a current detecting bridge through a circumferentially insulated surge resistor and from there to the common terminal of the aforementioned voltage source to thereby return one of the plates of the ion detecting device to the voltage source.
U.S. Pat. No. 4,440,995 discloses a vacuum circuit interrupter which utilizes the vapor deposition shields thereof and the existing high voltage electrical source or network which is controlled by the circuit interrupter to produce a cold cathode detector for determining the quality or amount of vacuum within the vacuum circuit interrupter. The central shield support ring which protrudes through the insulating casing of the circuit interrupter is utilized to supply electrical current to a current measuring device and to return one of the shields of the cold cathode detector to the common terminal of the aforementioned voltage source.
U.S. Pat. No. 4,491,704 discloses a vacuum monitoring device for use in vacuum circuit interrupters comprising a stacked resistor assembly as a voltage divider coupled to an internal shield of the vacuum bottle and a low voltage detection circuit for monitoring leakage currents under abnormal pressure conditions.
U.S. Pat. No. 4,937,698 discloses a system for foreseeing deterioration in interrupting a performance of a vacuum interrupter, including a first measuring component for measuring potentials of electric lines connected to fixed and movable electrodes of the vacuum interrupter; a second measuring component for measuring a potential of an arc shield; a signal transmitting section for the transmission of potential signals resulting from the measurements in the first and second-measuring component; a comparing section for making a comparison between the measured signal from the first measuring component and the measured signal from the second measuring component both transmitted through the signal transmitting section; and a judging section for judging that the fixed and movable electrodes have been deteriorated in their interrupting performance, on the basis of the result of the comparison made in the comparing section.
U.S. Pat. No. 5,286,933 discloses a vacuum circuit-breaker including, for each phase, at least one vacuum bottle housed inside a closed enclosure, wherein said circuit-breaker includes at least one scintillation fiber disposed in the space between said enclosure and the outside surface of the vacuum bottle(s), said fiber being connected outside the circuit-breaker to an opto-electronic device.